1. Field of The Invention
The present invention relates to apparatuses and methods for applying screens to cathode ray tube (CRT) or other video display faceplates. More particularly the present invention relates to direct contact printing of screens on CRT or other video display faceplates.
2. Discussion of the Related Art
As is known in the art of making color CRT screens, the interior of the glass faceplate, or screening surface, is coated with a black grille, also called a masking or surround, which forms a pattern of uncovered screen areas which are transparent, and surrounding areas which are opaque. Red-light-emissive, green-light-emissive, and blue -light-emissive phosphors are deposited over the black grille to form colored-light emitting areas of known dimension. Together this black grille and the phosphors comprise the basic screen, or image producing area of the CRT. The black grille, being laid down first and thereby determining the areas of light transmission of the screen, is thus critical to the imaging performance of the CRT. The technique of laying down the grille first allows greater tolerance in the phosphor dot or line placement as well as greater screen brightness and/or increased contrast.
Also, as is known, the screen pattern must be placed in registry with a shadow mask which determines the electron beam landing areas used to excite the light-emissive phosphors. "Registry" or "registration" as used herein will refer to the proper placement of elements of the CRT as necessary for the designed functioning thereof. As used herein the term "printing", as in "printing a screen", will refer generally to any kind of screen element deposition on to the faceplate. "Direct contact" or "contact" printing will refer to printing where a surface containing the screen elements, or their precursors such as tacky dots or the like, actually contacts the faceplate screening surface. "Offset" printing will refer to printing wherein the screen element or precursor will be transferred to an intermediate surface before transfer to the faceplate.
As an example, a current 14 inch (diagonal measure) CRT screen of the high-resolution type, commonly used as a computer monitor, has over 880.times.10.sup.3 triads of red, green, and blue light-emissive phosphor dots, each triad corresponding to one hole in the shadow mask. Thus there are about 2.6.times.10.sup.6 individual phosphor dots, at a pitch, or center to center, spacing of about 0.28 mm. Current manufacturing tolerances require tenths of a mil (one mil =0.001 inch) accuracy in phosphor deposition from the nominal dot position to accord adequate registry with the shadow mask.
Current manufacturing technique for screening, i.e. placing the screen elements, of the faceplate utilizes a shadow mask and screen mated-pair, wherein an individual shadow mask is used to serially photoexpose a chemically sensitized slurry coating on the screen area to form the grille and each of the three phosphor patterns. This process is described in U.S. Pat. No. 3,973,964 issued to Howard G. Lange and owned by the assignee hereof. Also described in the Lange patent, and in commonly owned U.S. Pat. No. 4,902,257, is a non-mated method of photoexposure screening denominated as "near contact" printing which may be utilized as part of, or in conjunction with, the present invention. While this photochemical screening process is now highly refined and accurate, it is also lengthy in time and resource intensive, requiring great expenditure of floor space, machinery, labor, and materials such as the expensive phosphor compounds and water needed for the repeated washing of the screen during the process. The disadvantages of the mated screen and mask approach are well known, as is the desire for a useful system of interchangeable screens and masks.
A method of screening the faceplate which obviates some of the difficulties associated with the above-described photochemical screening process involves the use of a "silk screening" process whereby phosphor pastes are forced through apertures of a cloth or wire mesh onto the faceplate to form the screen. This method is described in U.S. Pat. No. 2,625,734, issued to Law; and U.S. Pat. No. 4,248,947, to Oikawa, which discloses the possibility of silk screening phosphors over a previously optically produced grille.
However, even with all process variables optimized in the silk screening process, each phosphor type must still be serially applied to the faceplate, thus slowing manufacturing throughput and adding to the expense of the CRT.
Great Britain Patent No. 2,052,148 issued to Sony Corporation suggests that a flat faceplate panel having no projections thereon may be printed by (silk) "screen printing or other printing processes" including a first optical deposition of the grille followed by subsequent printing of the phosphors. The Sony reference, however, does not teach the use or desirability of direct-contact printing, or any means therefore. The Sony reference is drawn to a flat faceplate having guide grooves for a shadow mask frame assembly therein, and more nearly states a desired result or use of this faceplate, and not a printing method or apparatus.
To obviate the need for separate application of the grille and each phosphor type onto the faceplate it has been proposed, as in U.S. Pat. Nos. 4,549,928 and 4,557,798 issued to Blanding et al., to print each screen element, i.e. grille and red, green, and blue light-emissive phosphors, onto the faceplate in one pass as a contiguous composite film by contact with a common elastomeric collector roller onto which the screen elements are placed in the form of tacky ink-compositions. However, in such a "one-pass" operation, the grille can no longer be used as an opaque substrate for the phosphors. Further, due to the difference in grille-ink and phosphor ink compositions, transfer of the contiguous-sheet screen elements is problematic.
Particularly, by laying down a black-grille, or mask, in the same layering height proportion as the phosphor elements in order to achieve a continuous composite film on the collector for one pass application to the faceplate two problems arise. First, the black grille can no longer be used as an opaque substrate beneath the phosphors to define and limit the boundaries of the phosphor "windows" in the faceplate. Therefore, tolerances must be held exceedingly close under the disclosed method. Second, due to the differences in the particulate composition of the grille and of the phosphors the optimal grille ink dot height is different from that of the optimal phosphor-dot thickness. Thus, the surface of a continuous composite film which is presented to subsequent transfer rollers and ultimately, the faceplate, is compounded of different height dots, which can make transfer problematic. If more grille-ink is used than necessary to equalize dot height, distortions in the grille pattern may occur during subsequent heating operations necessary for CRT manufacture, such as runout of the grille or "ink", i.e. separation of the grille from the phosphor leading to an undesired transparent "window" in the faceplate surrounding the phosphor elements.
Also, due to the close tolerances required in modern high-resolution CRT screens, positional accuracy of the screen elements, most notably the grille, is exceedingly important. Thus, the number of surface-to-surface transfers of the screen element ink-patterns is an important consideration in developing relatively economical and highly accurate screen printing equipment. However, a one pass application of all screen elements in the form of thermoplastic inks, as disclosed in the Blanding, et al. patents, necessarily requires double-offset printing apparatus, thereby increasing the number of surface-to-surface transfers.
The prior art, as evidenced by: U.S. Pat. Nos. 2,625,734; 3,973,964; 4,248,947; 4,549,928; 4,557,798; and GB Patent No. 2,052,148; has recognized the desirability of a screen-printing method which obviates the mated-mask, photoexposure method. However it is believed that several key considerations of screen printing remain unaddressed, including suitable methods and apparatuses which retain all the advantages of the black-grille screen element and which provide accurate and economical phosphor deposition thereon, and eliminate the need for extraneous mask and/or screen hardware such as may be accomplished through use of the flat tension mask technology developed by the assignee hereof.